Labelling of packages has been an ongoing requirement for centuries. As automation becomes evermore a fact of life, the label and its information content play an ever wider role in achieving automation. The information on the label may contain information relating to the contents of the package, the source or destination of the package, relevant purchase and transit data, etc. In many applications, it is desirable to use this information in the course of processing the package. For example, the part number of the contents may be used in inventory management or the destination address may be used in automatically sorting packages.
To achieve automation effectively, some form of machine readable code such as bar code is usually employed. This then requires the use of automatic reading equipment to determine the information content on the label. Further, in the normal case where the information cannot be preprinted on the package, it is highly desirable to include some form of automatic label printer and applicator. Furthermore, packages are usually processed by a continually moving conveyor rather than manually moved.
In certain cases, the objects to be labelled are all the same size and the labels can be placed in a known fixed spot on the package. For example, one can define a fixed X-Y location on the side of a box, register packages against one side of a conveyor, locate a printer, applicator and package sensor suitably to apply the label and subsequently similarly locate a scanner to scan this same X-Y region of the package and thus read the label. This approach may work in a manufacturing environment where there is a limited number of package sizes.
However, in the majority of applications, notably merchandising and transportation, packages come in all sizes and shapes from a variety of sources not under the direct control of the sorter and defining a fixed location becomes impossible. Further, packages in transport tend to rotate about their vertical axis as they pass through various stages of the conveyor, thus possibly changing the face side that they present to a scanner compared to the labelled side. Some packages can also tend to tumble (rotate about a horizontal axis), especially when subjected to rapid acceleration, but this can usually be controlled if the package is oriented in its most stable condition when it is first placed on the conveyor.
The optimum place to put a label is thus the top of a package, regardless of whether the reader is human or a machine. If the label is on the side, rotation of some of the packages will be required to find the label and read it. Such rotation of the package in order to read a label is awkward when done manually and very cumbersome to automate. Thus labelling the top and subsequently reading the label is easy to do manually, but heretofore has presented considerable difficulty when done automatically, especially in view of the considerable variation in package height frequently encountered.
A significant component in a automatic labeling system is the device which applies the labels, known as the applicator head. Previous applicator head devices used two single passage air lines and a single manifold. Vacuum was applied through a controllable valve to one air line and thence to the manifold to retain the label. When it was desired to apply the label, the first line was disconnected and the other air line was connected to a source of pressure. The air blowing through the single manifold then released the label. For short stroke systems this approach was satisfactory. In the applicator herein disclosed, this approach is unworkable. The valves required can be located in only one of two places, either stationarily mounted to the frame of the applicator assembly or carried along with the applicator arm. If stationarily mounted, the air line from the valve to the apply head becomes untenably long, being in excess of 8 feet in the instant embodiment. This makes for extremely sluggish response time and unreliable label application. Carrying external valves along with the applicator head results in excessive weight and poor applicator response.
The devices which position or move the applicator head present an additional set of problems. The objects to be labelled are traveling along a conveyor which can be moving at any speed. The applicator will require a finite time to move the apply head down to a position just above the package to be labelled, which time will vary with package height. During this first half of the applicator cycle time, the package will move a finite distance along the conveyor. This package motion must be accounted for in determining when to initiate the applicator cycle. The applicator cycle time is thus a variable as a function of package height. The package motion is a variable that is a function of the conveyor velocity during the apply time. The conveyor velocity can be measured directly and in most (but not all) cases can be assumed constant during any one apply cycle. Since the apply cycle must be initiated prior to its occurrence, the apply cycle time must be predictable in advance over the full range of package heights in order to account for package motion during the apply cycle properly. Any errors in height measurement, conveyor velocity measurement and actual apply cycle time will result in a label position placement different from that desired. Hence the motor and control system chosen to drive the applicator must not only be capable of achieving the necessary throughput but the position performance must be predictable over the full range of package heights.
It would seem at first glance that a rapid acceleration constant velocity motor such as a clutch brake system or a stepping motor would be ideal for the application but as it turns out this is not the case. If half the allowable cycle time (400 milliseconds) is allocated to the down stroke, then the average velocity must be 80 inches per second with no start or stop times considered. Allowing 50 milliseconds start time and 25 milliseconds stop time brings the velocity to 96 inches per second and requires 5 G's to start the arm and 10 G's to stop it. The travel distance during starting is 2.5 inches and that during stopping is 1.25 inches. A typical weight for the arm system would be 4 pounds or so (without solenoid operated air valves), requiring a start force of 20 pounds and a stopping force of 40 pounds. If a stepping motor is used, the step rate at the required torque usually has to be limited to under 1500 steps per second, resulting in a drive pulley radius of 8.5 inches and a torque requirement of 8.5 in * 40# * 16 oz/in/2=2700 inch ounces, not counting the torque required to accelerate the motor itself. In stepping motors, it is very difficult to keep the developed torque constant as the motor speed increases principally due to the switching time of the phases, hence the idea of a constant acceleration is not attainable. In addition, these requirements on the motor are almost physically unrealizable. Moreover, the extremely high G forces on the arm drive system during starting and stopping will result in very high stress levels on the bearings and cable, bringing about early failure of these items, not to mention the problems of primary and secondary resonances in the arm-motor spring mass system. Although a constant velocity system seems to be simple from the standpoint of predicting the cycle time, physical implementation is anything but simple.
Thus, labelling a moving object requires the ordering of many events, such as label printing and label applicator positioning for each package to be labelled, while the packages continue to move rapidly on the conveyor. The variability in package height, size and spacing, together with the varied data to be printed on the labels require significant system agility and responsiveness to keep pace with the flow of packages. The mere connection of individually available position detecting, printing, label positioning and label application devices, even if available for the specific task, cannot form an integrated system capable of responding to the varied requirements while matching the package conveyor flow volume typically encountered.